CA1103069A - Method of making magnetic powders - Google Patents
Method of making magnetic powdersInfo
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- CA1103069A CA1103069A CA306,504A CA306504A CA1103069A CA 1103069 A CA1103069 A CA 1103069A CA 306504 A CA306504 A CA 306504A CA 1103069 A CA1103069 A CA 1103069A
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Abstract
ABSTRACT OF THE DISCLOSURE
A method of making magnetic powders is disclosed which comprises the steps of dispersing iron oxide particles into aqueous solution of cobalt salt, adding alkali to the aque-ous dispersion to such an extent that its alkali concentra-tion becomes higher than 3 mol/? but lower than 12 mol/?, and heating a resultant dispersion so as to modify said iron oxide particles with cobalt, whereby magnetic powders which are superior in magnetic characteristics can be obtained.
A method of making magnetic powders is disclosed which comprises the steps of dispersing iron oxide particles into aqueous solution of cobalt salt, adding alkali to the aque-ous dispersion to such an extent that its alkali concentra-tion becomes higher than 3 mol/? but lower than 12 mol/?, and heating a resultant dispersion so as to modify said iron oxide particles with cobalt, whereby magnetic powders which are superior in magnetic characteristics can be obtained.
Description
1~3~69 BACKGROUND OF THE INVENTION
Field of the Invention The present invention relates generally to a method of making magnetic powders which have high coercive force values and improved rectangular ratios (Br/B ) and improved print-through characteristics when used in a magnetic recording medium and is directed more particularly to a method of making novel acicular iron oxide powders.
Description of the Prior Art Recently, magnetic iron oxide which includes cobalt CO and is suitable for high density recording has been investi-gated. As well known, when Co2+ ion is contained in acicular magnetic iron oxide, its magnetic characteristics are further improved due to the crystal anisotropy in combination with the shape anisotropy. On this point reference is invited to "Cobalt substitutedr-Fe2O3 as high Density Recording Tape" by IEEE Trans Action On Electronics Computers Vol. EC-15 No. 5 1966 and so on.
In general, there are three methods for producing y-Fe2O3 containing Co.
1. Ferric hydroxide containing cobalt hydroxide is subjected to water-heat treatment or processing in aqueous alkali solution, thus produced powders are reduced and then oxided into r-Fe2O3 containing Co.
Field of the Invention The present invention relates generally to a method of making magnetic powders which have high coercive force values and improved rectangular ratios (Br/B ) and improved print-through characteristics when used in a magnetic recording medium and is directed more particularly to a method of making novel acicular iron oxide powders.
Description of the Prior Art Recently, magnetic iron oxide which includes cobalt CO and is suitable for high density recording has been investi-gated. As well known, when Co2+ ion is contained in acicular magnetic iron oxide, its magnetic characteristics are further improved due to the crystal anisotropy in combination with the shape anisotropy. On this point reference is invited to "Cobalt substitutedr-Fe2O3 as high Density Recording Tape" by IEEE Trans Action On Electronics Computers Vol. EC-15 No. 5 1966 and so on.
In general, there are three methods for producing y-Fe2O3 containing Co.
1. Ferric hydroxide containing cobalt hydroxide is subjected to water-heat treatment or processing in aqueous alkali solution, thus produced powders are reduced and then oxided into r-Fe2O3 containing Co.
2. In case of making acicular goethite, goethite, containing CO
is produced by adding an aqueous cobalt salt solution to adjust the pH, then reduced and oxidized to produce y-Fe2O3 containing Co .
is produced by adding an aqueous cobalt salt solution to adjust the pH, then reduced and oxidized to produce y-Fe2O3 containing Co .
3. Goethite containing no CO is used as a nucleus, a reaction the same as the method 2 is carried out on the nucleus to grow goethite containing CO~ and then this goethite is reduced and oxidized to produce y-Fe2O3 containing CO.
11~3~69 According to the above prior art methods, it is pos-sible to control the coercive force of the powders over a wide range by adjusting the amount of C contained therein, but there are defects such as the manufacturing Processe~ thereo-~ are rather complicated, the powders thus produced are not stable, being reduced much in magnetic characteristics by pressure and heat and being poor in print-through characteristics. These defects may be explained by the phenomena that the crystal magnetic aniso-tropy of 3 axes directions of the cubic lattice becomes dominant due to the diffusion of Co2+ ion entered into 16d site of the spinel crystal structure.
The assignee of this application filed a patent appli-cation on a novel method free from the prior art defects which was laid open as Japanese patent application publication No.
10994/73, which is now considered effective. The method of this publication is generally referred to as a cobalt hydroxide adsorbing method, in which cobalt hydroxide is adsorbed on the surface of acicular goethite, y-Fe2o3 or Fe3O4 and when the nucleus is y-Fe2O3 an acicular magnetic material of high coer-cive force can be obtained only by subjecting the same to asuitable thermal heating process. According to various experi-ments, however, when r-Fe2O3 adsorbed with cobalt hydroxide is subjected to a thermal treatment or processing at, for example, high temperature (400C) to diffuse in the particles thereof Co2+ ion, the coercive force increases remarkably but the insta-bility of its magnetic characteristics becomes great and its print-through property becomes worse than that of the original material. Further, even when Co2+ ion is not diffused in the particles, when CO-ferrite appears on the surface of the particle, the print-through property is also deteriorated.
It is also known that iron oxide powders adsorbed with 3Gi~9 - cobalt hydroxide as such are used in the magnetic medium. Iron oxide magnetic powders of this type are disclosed in the Japanese Publication Nos. 74399/74 (Toda Kogyo Ltd., Co.), 74400/74 (Toda Kogyo Ltd., Co.), 113159/74 (Tokyo Denki Kagaku Kogyo Ltd., Co.) and so on. According to the methods of making iron oxide magnet-ic powers disclosed therein, alkali is added to an aqueous solu-tion of cobalt salt, into which iron oxide powders are dispersed, to deposit a cobalt compound on the surface of iron oxide pow-ders. In this case, the concentration of alkali is lower than 3 mol/~e. The thus obtained iron magnetic powders have the depos-ited cobalt hydroxide on their surface, so that they have a large hydrophilic property due to OH radicals on their surface. Thus, their dispersion property in organic solvents is low which is necessary during magnetic paint manufacturing process in magnetic recording medium making process, and hence the rectangular ratio Br/Bm of the magnetic recording medium is low. Further, even if the iron oxide magnetic powder which is made by the above method is subjected to thermal treatment to decompose the cobalt hydroxide on the surface of powder and to decrease the hydro-philic property on the surface and hence to increase the disper-sion property in the paint, the increase in dispersion property is not sufficient when the alkali concentration is lower than 3 mol/~, because the deposition of cobalt compound on the surface of iron oxide powder is not homogeneous. Further, the iron oxide magnetic powder, on which a cobalt compound is deposited from a solution whose alkali concentration is lower than 3 mol/~, is lacking in improvement of its coercive force Hc with respect to the depositing amount of cobalt compound. If high coercive force is desired to be obtained, it is necessary to deposit a large amount of cobalt compound, which results in the iron oxide magnetic powder being lowered in magnetization degree and hence ' ' . . ., - ~
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a magnetic recording medium made by using such a powder is low in output.
OBJECTS AND SUMMA:RY OF THE INVENTION
The present invention has as an object to provide a method of making a magnetic iron oxide powder which is high in coercive force for use with a novel magnetic recording medium which is improved in print-through characteristic and high in rectangular ratio Br/Bm.
It is another object of the invention to provide a method of making a magnetic iron oxide powder in which iron oxide is dispersedin an aqueous solution of cobalt salt, alkali is added to this solution to such an extent that the alkali con-centration of the liquid phase the reacted dispersion is higher than 3 mol/Ae but lower than 12 mol/A~ and the dispersion is heated to deposit~a cobalt compound on the surface of iron oxide particles to modify the properties thereof~
A further object of the invention is to provide a method of making magnetic iron oxide particles in which the magnetic iron oxide powder having deposited thereon the above modified cobalt compound is subjected to thermal treatment in a non-reduction atmosphere at a temperature between 100C and 200C.
According to an aspect of the present invention there is provided a method of making magnetic iron oxide powder which comprises the steps of dispersing iron oxide particles into an aqueous solution of a cobalt salt, adding alkali to said aqueous solution to such an extent that the alkali concentration of the liquid phase of the reacted dispersion becomes higher than 3 mol/~ but lower than 12 mol/~ , and heating the resultant dis-persion so as to modify said iron oxide particles with cobalt.
The other objects, features and advantages of the ~ 3~6i9 present invention will become apparent from the following de-scription taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF T~E DRAWINGS
Figs. 1 to 5 are graphs showing the relations between the coercive force of magnetic powders and their thermal treat-ment or processing time used for explaining the method of the present invention;
Fig. 6 is a graph plotting the coercive force against the atomic ratio of cobalt to iron of magnetic powders which are used for the explanation of the invention;
Fig. 7 is a graph showing the relation between the coercive force of magnetic powders and their thermal processing temperature;
Fig. 8 is a graph showing the relation between the rectangular ratio of a magnetic tape and the thermal processing temperature of magnetic powders; and Fig. 9 is a graph showing the relation between the print-through value of a magnetic tape and the thermal processing ` temperature.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method of the present invention will be described hereinbelow. According to the present invention, acicular iron oxide particles such as r-Fe2O3, Fe3O4 and a substance whose oxidized state is therebetween, i.e., O~Fe2+ /Fe3+ ~0.5 are dispersed into an aqueous solution of cobalt salt; alkali is added to this aqueous solution to such an amount that the alkali concentration of the liquid phase of the resultant reaction dis-persion becomes more than 3 mol/~but less than 12 mol/A?; and the resultant reaction dispersion is heated to modify the acicular iron oxide particle with cobalt. That is, the surface of the acicular iron oxide particle is coated with a cobalt compound ~3q~
consisting of cobalt hydroxide, cobalt oxide or an intermediate product. Then, the reaction compound is washed by water, dried at a predetermined temperature, rinsed with water again, and dried again. Thereafter, the product is subjected to a heating process at a predetermined temperature in non-reducing atmos-phere to produce desired magnetic powders.
In case of making the magnetic powders, it is desired that the mixing ratio of acicular iron oxide particles with cobalt salt be such that the atomic ratio of cobalt with iron, Co/Fe, of the resultant particle is within the range between 0.1 and 10 at. %. As the aqueous solution of cobalt salt, an aqueous solution of cobalt chloride, cobalt bromide, cobalt sulfate, cobalt nitrate, cobalt acetate, or mixtures of two or more of the foregoing or the like can be employed. Also, the alkali which is used in the present invention is a strong alkali or hydroxide of alkali metal such as lithium hydroxide, sodium hydroxide, potassium hydroxide, or mixtures of two or more of the foregoing or the like. The alkali concentration of the liquid phase of the resultant reaction dispersion, which consists of the aqueous solution of cobalt salt dispersed with acicular iron oxide particles and treated with alkali, is select-ed between 3 mol/~ and 12 mol/~, as set forth previously, and more desirably between 3.5 mol/~ and 10 mol/~. When the said alkali concentration becomes lower than 3 mol/~, the coercive force Hc becomes low, the dispersion property is deteriorated, and the rectangular ratio of a magnetic recording medium using the par-ticles is lowered. While, when the said alkali concentration exceeds 12 mol/~, the surface of the iron oxide particle is partially dissolved and its shape is deformed, which results in that the increase of coercive force Hc is ended and the rectangu-lar ratio becomes low. In this case, the alkali concentration is defined as the concentration of the ion pair of alkali metal ' -6-~ ., ~3Ç~
atom cation and hydroxy ion in the liquid phase of the resultant reaction dispersion.
When the acicular iron oxide particles are dispersed into the aqueous solution of cobalt salt, it is desired that the acicular iron oxide particles be dispersed into an aqueous solution of cobalt salt whose weight is same as or greater than that of the acicular iron oxide particles and that the aqueous solution of cobalt salt have a concentration lower than 1.5 mol/~, more desirably lower than 1.0 mol/~ so as to deposit a cobalt compound homogeneously on the surface of the acicular iron oxide particles. The heating temperature of the dispersion is generally desired to be between 70C and the boiling point thereof, and as the atmosphere outside the dispersion, a non-oxidizing atmosphere such as nitrogen, argon, mixture of the foregoing or the like can be used, but an oxidizing atmosphere such as oxygen, air, or a mixture of the foregoing with nitrogen or argon or the like therewith are more desirable.
The acicular iron oxide particles having a cobalt compoun~
deposited thereon are washed with water, dried at a predetermined temperature, or after being washed with water and dried, they are subjected to a heating process at a predetermined temperature in non-reducing atmosphere to achieve desired magnetic powders or parti-cles. In this case, the range of temperatures at which the particles, which have been dried and are subjected to the heating process in non-reduction atmosphere, is desirably between 100C
and 200C, more desirably between 120C and 200C. When the temperature exceeds 200C, the coercive force Hc of magnetic particle is lowered, while when the temperature becomes lower than 120C and further lower than 100C, the dispersion property thereof becomes poor. The print-through characteristic is good at the temperature range between 100C and 200C, more desirably ~3~
between 120C and 200C, and becomes poor when outside the above temperature ranges. The heating or thermal process time in non-reducing atmosphere is required more than 0.5 hours, but if the heating process time is carried out in more than 5 hours at the high temperature side (~200C), the coercive force Hc becomes low, which is not desired. As the atmosphere for the thermal process, oxidizing, inert and reducing atmospheres may be used, but if the coercive force Hc, dispersion property, print-through characteristic and so on are taken into account, oxidizing and inert atmospheres are desired. Especially, in order to change Co(OH)2 on the surface of the magnetic powder (master powder) to oxide easily and stably without causing any lowering of the magnetic characteristic, an oxidizing atmosphere is most desired.
If a reducing atmosphere such as hydrogen gas is employed, cobalt ferrite appears partially and hence the print-through characteristic is deteriorated. In the case of inert atmosphere i.e. nitrogen gas, when compared with air, the dispersion proper-ty i.e. rectangular ratio Br/Bm and print-through value are lowered somewhat but are better than those of the prior art.
In the case that alkali is mixed into the aqueous solution of cobalt salt, into which iron oxide magnetic particles are dispersed, when the mixture is heated to deposit cobalt com-pound on the surface of the magnetic powders and magnetic powders of high coercive force are made, it is generally said that as the amount of added cobalt is great, the coercive force of mag-netic particles is high. However, the inventors of the present invention have found that the rectangular ratio Br/Bm of a magnetic tape using the magnetic powder, which is prepared under the condition of an alkali concentration between 3 mol/~ and 12 mol/~e regardless of the amount of added cobalt, is improved.
As set forth above, according to the invention, the magnetic record~ng med~um which is high in coercive force, superior in dispersion property and good in print-through effect is made. In the invention, even if Co2+ ion is not diffused into iron oxide particles, the coercive force Hc becomes high. The reason for this may be that some magnetic interaction appears on the boundary between the surface of the iron oxide particle and the adsorbed substance i.e. cobalt oxide (surface magnetic anisotropy).
Hereinbelow, the present invention will be described further with reference to Examples.
Example 1 Acicular y-Fe2O3 particles (whose coercive force Hc is 380 e~ whose longer axis is 0.5 ~m (micrometer) and whose axis ratio is about 8) in an amount of 3 Kg had been dispersed into 20Q of water, 2Q of cobalt salt aqueous solution in which 894g of CoCQ2 6H2O had been dissolved, was added to the former, and the resultant mixture was stirred sufficiently, which means that Co was added at 10 at. ~ (Co/Fe ratio).
Then, 8Q of alkali aqueous solution in which 3.8 Kg of NaOH
had been dissolved was added to the above dispersion. The resultant dispersion was in an aqueous medium which had a strong alkali concentration of about 3 mol/Q. The final dispersion was heated at 100C for 4 hours while being stirred sufficiently. After the heating, the dispersion was rinsed with water to have a neutral pH, and filtered with a suction filter. Thus, ~-Fe2O3 particles, each having Co on the surface thereof, were produced. These particles were dried and then subjected to a thermal processing at 100C in air for 5 hours. The magnetic characteristics of magnetic particles thus produced were as follows;
Saturated magnetization as = 71.8 emu/g Coercive force Hc = 675 e g _ 3~
Ratio ~r (residual magnetism~/sS = 0.48 The thus produced iron oxide powders, each con-taining Co, were mixed with the following composition for about 48 hours in a ba~l mill to produce magnetic paint.
Magnetic iron oxide powders 100 wt. parts containing Co Vinyl chloride-vinyl acetate 17.5 wt. parts copolymer (Binding agent)(VAGH: Trade name of UCC Ltd, Co.) Polyurethane Resin 7.5 wt. parts (Binding agent)(Estane 5702: BF Goodrich Chemical Co.) Lecithin (Dispersion agent) 2.0 wt. parts Methyl-ethyl ketone (Solvent) 100 wt. parts Cyclohexanone ~Solvent) 100 wt. parts The above magnetic paint was coated on a film, which is made of polyethylene terephthalate with 12 ~m in thickness such that the thickness of the paint after being dried is 6 ~m. Thus, a magnetic tape was produced. In this case, the coercive force Hc thereof was 660 e and the rectangular ratio Br/Bm thereof was 0.79.
Figs. 1 to 5 are graphs, respeativel~, showing the relations between the coercive force Hc of the magnetic powder and vary;ng Co adsorbing conditions.
Fig. 1 is a graph showing the relation between the coercive force Hc of the magnetic powder and the heating time (hr) of the reaction dispersion under the condition that the added amount of Co was 1 at. % and NaOH concentrations of the reaction dispersion (1 mol/Q, 3 mol/Q, 5 mol/Q, 10 mol/Q
and 15 mol/Q) were varied.
Figs. 2 to 5 are graphs showing the characteristics 3Q similar to that of Fig. 1 when the added amount of Co was 3 at. %, 5 at. %, 10 at. %, and 15 at. %, respectively. In the respective graphs, the marks X,~;O ,~ and ~ show the cases -- 10 _ ~ ~3~
of NaOH concentration of 1 mol~, 3 mol/Q, 5 mol/Q, 10 mol/Q
and 15 mol~Q, respectively.
The following Tables I, II, III and IV are respectively made from the characteristic graphs of Figs. 1 to 5 and show the changes of the coercive forces and rectangular ratios of magnetic tapes which are prepared by using the magnetic powders- made by varying one of Co adsorbing con-ditions.
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Table I
_ Co absorbing condition Concent- Adding Heating Heating Coercive Rectan-ration of amount temper- time force of gular NaOH of Co ature tape ratio (at.~, of tape (mol/") Co/Fe) (C) (hour) (e) (Br/Bm) Comparison 1 10 100 4 528 0.73 _ __ _ Example 3 10 100 4 660 0.79 Example 5 10 100 4 710 0.80 . _ 3 10 10 100 4 750 0.81 .
~ Dn ; 15 ~ ~ ~ o l o ~l ~3~
,:
~able II
. . Co absorbing condition Concent- Adding Heating Heating Coei-cive Rectan-ration amount temper- time force of gular of NaOH of Co ature tape ratio (at. %, of tape (mol/Q) Co/Fe) (~C) (hour) (e) (Br/Bm) ._.. _ . Example 3 1 100 1 470 0.79 _ _ _ .~ _. _. _ .
Example 3 5 100 1 575 0.79 ... __ .....
6 3 10 100 1 608 0.79 . __ ..
Comparison 1 1 100 1 447 0.72 .
Comparison . 4, 1 5 100 1 488 0.74 . .
_ _ _ __ Comparison 1 10 100 1 491 0.73 .
~ 13 -1~3069 Table III
.~
Co absorbing condition Concent- Ac-ding Heating Heating Coercive Rectan-ration amount temper- time force of gular of NaOH of Co ature tape ratio (at.%, of tape (mol/Q) Co/Fe) (C) (hour) (e) (Br/Bm) _ _ Example 35 100 1 575 0.79 _ _ __ _ _ .
Exa8mPle 3 5 100 4 658 0.80 .. __ _ _ _ Example 3 5 100 24 690 0.82 .. .; _ . . . . _ ._ ._ Comparison 1 5 100 1 488 0.74 _ _ I .
Comparison 7' 1 5 100 4 513 0.75 .. .. _ . . ._ . .
Comparison 1 5 100 ~ 549 0.76 ~3~69 Table IV
Co absorbing condition . Concent- Adding Heating Heating Coercive Rectangu-ration amount temper- time force of lar rati of NaOH o(atC,~o, ature tape of tape (mol/Q) Co/Fe) (C) (hour) (e) (Br/Bm) Comparison 1 10 100 24 580 0.74 _ Example 3 3 100 4 576 0.80 ~
_ ...
Example 5 3 100 1 580 0.81 Example 10 3 100 0.5 586 0.81 _ Comparison15 3 100 0.5 570 0 76 3~
Table I shows the characteristics of a tape which is made by varying the amount of sodium hydroxide in aqueous medium, with all other conditions being held constant.
In Table I, Examples 1, 2 and 3 are the cases that the NaOH concentration is at 3 mol/Q, 5 mol/Q and 10 mol/Q, respectively, and Comparisons 1' and 2' are the cases that the NaOH concentration is at 1 mol/Q and 15 m~l/Q, respectively.
Further, in Table I the reason why the rectangular ratio of Comparison 1' in which NaOH concentration 1 mol/Q, is low may be explained that under this condition Co is not adsorbed on the surfaces of magnetic powders homogeneously, and the reason why the rectangular ratio of Comparison 2', in which NaOH concen-tration is selected 15 mol~Q, is low may be considered that under this condition the magnetic powders are partially dissolved due to the high alkali content and hence their physical forms are deformed or poor. In this case, it is ascertained that even ~f the concentration of NaOH in the reaction solution was higher than 12 mol/Q, no magnetic powders of higher coercive force could be obtained.
The above Table II shows the characteristics of respective tapes in which the amounts of Co and NaOH were varied but the other conditions were kept constant. In Table II, the Examples 4, 5 and 6 show the cases that the NaOH concentration is held constant at 3 mol/Q but the adding amount of Co is varied 1 at. %, 5 at. % and 10 at. %, respectively, while Comparisons 3', 4' and 5' show the cases that the NaOH concentration is held constant at 1 mol/Q but the adding amount of Co is selected 1 at. ~, 5 at. % and 10 at. % respectively. From Table II it is noted that, regardless of the added amount of Co, in case of making magnetic powders with a NaOH concentration higher than 3 mol/Q, the rectangular ratio of a magnetic tape increases.
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The above Ta~1e III shows the characteristics of a magnetic tape in which the heating time of the reaction dispersion and NaOH concentration therein are varied but the other conditions are kept the same. In Table III, Examples 7 (~which is the same as Example 5), 8 and 9 show the cases that NaOH concentration is held constant at 3 mol/Q and the heating time is 1, 4 and 24 hours, respectively, while. Comparisons 6' (which is the' same as Comparison 4'), 7~ and 8' show the cases that NaOH concentration is held constant at 1 mol~Q and the heating time is 1, 4 and 24 hours, ' '.
respectively. From Table III it will be noted that, in the case where magnetic powders are made wi.th an NaOH concentration higher tI.an 3 mol~Q, the rectangular ratio of the magnetic tape increases regardless of the heating time.
When such a relation between NaOH concentration and the coercive force of produced magnetic powders is con-sidered, as NaOH concentration becomes lower than 3 mol/Q, the coercive force of the produced magnetic powders becomes low, and also even if the added amount of Co is increased and the heating time is increased under the same condition, the coercive force is not increased. Accordingly, in order to obtain a coercive force higher than a coercive force HC-600 e' it is necessary to use a NaOH concentration higher than 3 mol/Q.
Further, even if the magnetic powder of the required coercive force can be produced with a NaOH concentration of lower than 3 mol~Q, it is hetter, in view of the rectangular ratio of magnetic tape, to reproduce under the condition of NaOH concentration higher than 3 mol/Q, selecting the condition of the adding amount of cobalt and the heating time.
The above Table IV shows the rectangular ratios of magnetic tapes using magnetic powders with the coercive ~ 3~
force of about 580 e which are made by varying NaOH con-centrations. In Table IV Comparison 9', Examples 10, 11~
12, and Comparison 10' are the cases that NaOH concentration was 1 mol/Q, 3 mol~Q, 5 mol~Q, 10 mol~Q and 15 mol/Q, respectively. From Table IV, it will be noted that the magnetic powders, wh~ch are produced with a NaOH concentration 3 to 10 mol/Q, have good rectangular ratios when they are used to form magnetic tapes and such magnetic powders can be produced with smaller amounts of cobalt and shorter heating times. As described above, in order to improve the rectangular ratio of a magnetic tape, it is preferred that NaOH concen-tration during coba;lt adsorption is within a range of 3 mol/Q
to 12 mol~Q.
Fig. 6 is a graph showing the coercive force of magnetic powders at respective Co adding amounts with varying NaOH concentrations. The graph of Fig. 6 is prepared from those of Figs. 1 to 5 in which only their maximum values are extracted and in which the values inscribed in the vicinit~ of the respective marks represent the heating time in hours. From the graph of Fig. 6 it will be noted that if NaOH concentration is maintained within the range of the present invention the coercive force increases and if the added amount of Co is lower than 10 at. % in the atomic ratio of Co~Fe, the improved results are achieved.
Example 13 3 Kg of acicular ~-Fe2O3 powders or particles (having a coercive force HC=380 e' long axis of 0.5 ~m and axis ratio of about 8) had been dispersed into 20Q ot water, aqueous solution of 2Q into which 447 g of CoCQ2-6H2O
(on market) had been dissolved was added to the former and then the mixture dispersion was stirred sufficiently, which resulted in a dispersion which contained Co at 5 at. % (in 3q~
Co/Fe atomic .ratio~. Then, 8Q of aqueous solution into which6.0 Kg of NaOH has l~een dissolved was added to the above resultant dispersion, so that the finally resultant suspension had a solution of strong alkali of about 5 mol/~. This final dispersion was heated at 100C for about 1 hour while being stirred sufficiently. After heating, the solution was rinsed with water to be neutral in pH, y-Fe2O3 particles on which cobalt hydroxide was deposited were extracted by means of a suction funnel, and then the y-Fe2O3 particles were dried. The 10 magnetic characteristics of thus obtained magnetic particles were such that saturation magnetization ~s was 73.3 emu/g, coercive force Hc was 606 e and ~r (residual magnetization)/aS
was 0.48, respectively. These magnetic particles will be referred as a master powder A.
The master powder ~ was subjected to thermal processing in air with the temperature being changed from 70C
to 400C. Then, when the surface condition of the magnetic particle or Co adsorption condition was analyzed by the electron ray d~ffraction and X-ray photoelectron spectrometry 20 (ESCA), the following Table V was obtained.
Table V
State of Co adsorbed on magnetic particle Thermal processing (8y Electron ray difraction and condit'ion ' ' X-'ray photoelectron spectrometry~
70C - 20 hours Co(OH)2 100C - 1 hour Co(OH~2 130C - 1 hour CoOOH, Co3O4 150C - 1 hour CoOOH, Co3O4 150 C - 5 hours 3O4 200C - 1 hour Co3O4 370C - 1 hour C3O4, CoFe2O4 400C - 1 hour CoFe2O4 ~3~
According to this Table V, when the temperature is lower than 100C, Co~OH~2 on the surface of the master powder A is still as it is or not changed. Butf when the temperature goes up to 130C, it is observed that CotOH)2 is changed into Co3O4. It is ascertained that as the temperature becomes higher, Co is gradually diffused into the master powder A.
The following Tables VI and VII respectively show the results when the master powder A was subjected to the thermal processing in nitrogen and hydrogen atmospheres and a similar analysis was carried out on the resultant product. According to the thermal processing in nitrogen, it is noted that the adsorbed cobalt hydroxide is changed over about 130C, and as the temperature of the thermal processing is raised further, Co starts its diffusion into the magnetic powder. While, according to the thermal processing in the hydrogen atmosphere, it is noted that Co starts its diffusion into the magnetic powders at the temperature of about 200C and at the same time the master powder starts to be reduced.
Fig. 7 is a graph showing the measured results of a coercive force Hc f the magnetic powder after the similar thermal processing has been carried out for about 1 hour. In the graph of Fig. 7, a curve I connecting the marks O , a curve II connecting the marks X and a curve III connecting the marks ~ show the thermal processes in the atmospheres of air, nitrogen and hydrogen, respectively.
It is noted in the thermal processes of air and nitrogen atmospheres that the coercive force Hc is lowered considerably within the temperature range of 200C to 350C, which may be due to the fact that as the temperature of the thermal process becomes high, Co2~ ion is caused to be moved and the ~3~
structure of the interface between the magnetic particle and the substance adsor~ed thereon, which interfacial structure causes the increase of t~e coercive force Hc, will disappear.
As the temperature of the thermal process becomes higher, the coerci~e force Hc again increases, which is caused by the fact that Cois diffused into the magnetic powders. Since the similar diffusion of Co occurs at relatively low temperature in the thermal process in the hydrogen atmosphere, it may be considered that no temperature range within which the coercive force Hc is lowered has been observed.
~ .
'~
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Table VI
State of Co adsorbed on magnetic Thermal processing (By electron-ray diffraction condition and X-ray photoelectron spec-trometry) ... ..
130C - 1 hour CoOOH, Co(OH)2 150C - 1 hour CoO, CoOOH
150C - 5 hours CoO
200C - 1 hour CoO
300C - 1 hour CoO, CoFe2O4 400C - 1 hour _ .
Table VII
. .
State of Co adsorbed on magnetic Thermal processing (By electron-ray diffraction conditlon and X-ray photoelectron spec-trometry) . . _ __.
150C - 1 hour Co(OH)2 200C - 1 hour CoO, CoFe2O4 300C - 1 hour 2 4 ,~ .
~ ~, ~i3~
Fig. 8 is a graph showing the results obtained when the magnetic powders, which are subjected to the thermal processes in the above respective atmospheres, are used to provide magnetic tapes and the rectangular ratios (Br/Bm) of the respective magnetic tapes are measured. In the graph of Fig. 8, a curve rv connecting the marks ~, a curve V connect-ing the marks ~ and a curve VI connecting the marks ~ represent the cases of the thermal processing in air, nitrogen and hydro-gen atmospheres, respectively. In this case the magnetic tape is made by the same manner as that of Example 1.
From the graph of Fig. 8 it is noted that the cobalt oxides derived from cobalt hydroxide which is coated on the surface of the master powder is superior in the dis-persion property.
Fig. 9 is a graph showing themeasured print-through values of the magnetic tape having the characteristics of Fig. 8 according to JISC-5542. In the graph of Fig. 9, a curve VII connecting the marks O , a curve VIII connect-ing the marks ~ and a curve IX connecting the marks ~ re-present the thermal processes in air, nitrogen and hydrogen atmospheres, respectively. It is noted that in air and nitrogen atmospheres, the print-through value begins to be improved with the thermal processing at temperature higher than 100C and further about 120C. While it is noted that when the temperature of the thermal processing becomes high, the print-through value is deteriorated due to the diffusion of Co into the master powder. In the graph of Fig. 9, the print-through value of the tape lower than -50 dB can not be used as a practical tape.
From the above resul~s, it will be understood that magnetic particles, which will represent superior characteristics when coated on a tape base, are obtained in the case that they are subjected to the thermal processing in air or inert gas at the temperaturé range between 100C and 200C, and preferably 120C and 200C.
It is also possible that even if magnetic particles of Fe3O4 or a substance whose oxidizing condition is be-tween Fe3O4 and Fe2O3 (intermediate substance), are used, such magnetic particles whose surface is covered by cobalt oxide can be made and hence a magnetic tape having improved characteristics is obtained.
Example 14 2 Kg of magnetic particles, whose divalent to triva-lent iron ratio Fe2~Fe3~ is 0.20 (which have the coercive force Hc of 260 e~ long axis of 0.5 ,um and axis ratio of about 8) had been dispersed into 20Q of water, 2Q of aqueous solution into which 300g of CoCQ2 6H2O had been dissolved was added to the dispersion, and the resultant suspension was stirred sufficiently. Then, 8Q of aqueous solution into which 6.0 Kg of NaOH had been dissolved was added to the dispersion, and the resulting dispersion was heated at 100C
for 1 hour while being stirred sufficiently. The magnetic characteristics of the particles are as follows.
~s = 80.2 emu/g Hc = 576 e ar~aS = 0.46 This magnetic powder will be referred as a master powder B.
The following Table VIII shows the characteristics of the master powder B after being subjected to thermal processing at 150C for 1 hour in air and those of a magnetic tape which is made by using thus prepared magnetic powders in the manner as recited in Example 1. From Table VIII
it is noted that the characteristics of the tape are improved by the thermal processing.
Example 15 3 Kg of ~-Fe2O3 particles (having a coercive force Hc = 405 e~ long axis = 0.4 ,um and axis ratio of about 8) had been dispersed into 20Q of water, aqueous solution into which 268g of CoCQ2-6H2O (on market) had been dissolved was added to the dispersion, and the resulting dispersion was stirred sufficiently, which results in that about 3 at. ~ (Co/Fe atomic ratio) of Co was added. Then 8Q of aqueous solution into which 4.2 Kg of NaOH had been dissolved was added to the above dispersion. Thus, the finally resultant liquid phase had a strong alkali content of about 3.5 mol/Q.
This resulting dispersion was heated at 100C for 1 hour while being stirred sufficiently. Thus prepared magnetic powders have the magnetic characteristics that their ~s = 74.6 emu/g, Hc = 587 e and ~r~s = 0.48. These magnetic powders will be referred to as a master powder C.
The magnetic characteristics of a magnetic tape which uses magnetic powders prepared by subjecting the master powder C to the thermal processing in the manner similar , .
l~C3~
to that of Example 1 are shown in the following Table IX.
Table IX
Thermal process~ng Coercive Rectangular Print-through condition force of ratio of value of magnetic tape tape powd~Or~ Br/B (dB) None 569 0.74 -51.1 Air 130C - 1 hour 563 0.83 -56.3 Nitrogen atmosphere 200C - 1 hour 558 0.82 -54.7 From the above Table IX is is ascertained that the characteristics of the magnetic tape are improved by the thermal processing.
Also, according to the method of the present invention the master powders can be made by using substances different from the cobalt salt and alkali used in Example 13.
Example 16 3 Kg of acicular y-Fe2o3 particles (having the coer-cive force = 380 e' long axis = 0.5 ,um and axis ratio = 8) were dispersed into 20~ of water, 2Q of aqueous solution into which 528 g of CoSO4 7H2O (on market) had been dissolved was added to the dispersion, and the resultant dispersion was stirred sufficiently, which meant that Co was added at about 5 at. % (Co/Fe atomic ratio). Next, 8~ of aqueous solution into which 6.3 Kg of LiOH H2O had been dissolved was added to the above aqueous dispersion. Thus, the final dispersion had a strong alkali solution of about 5 mol/Q.
The magnetic characteristics of magnetic powders, which were obtained by processing the above solution in the manner similar to that of Example 13, are as follows. These are ~s = 74.2 emu/g, Hc = 628 e and ~r/~s = 0.48. This magnetic powder will be referred to as master powder D.
, 3~
The characteristics of a magnetic tape, which uses magnetic powders provided by subjecting the master powder D to the thermal processing similar to Example 13, are shown in the following Table X.
Table X
Thermal Processing Coercive Rectangular Print-through condition force of ratio of value of magnetic tape tape H Wd(or) Br/Bm (dB) None 628 0.75 -50.8 Air 160C - 1 hour 619 0.83 -55.1 Nitrogen atmosphere 200C - 0.5 hour 651 0.81 -49.0 From the above Table X it is noted that the dispersion property and print-through characteristics are especially improved by the thermal processing in air.
As described above, according to the method of making magnetic powders of the invention, a magnetic recording medium, which has high coercive force and superior disper-sion property and Is superior in print-through characteristic, can be made. This magnetic medium is preferred for use as a high density recording tape and so on.
It will be apparent that many modifications and variations could be effected by one skilled inthe art without departing from the spirits or scope of the novel concepts of the present invention so that the scope of the invention should be determined by the appended claims only.
11~3~69 According to the above prior art methods, it is pos-sible to control the coercive force of the powders over a wide range by adjusting the amount of C contained therein, but there are defects such as the manufacturing Processe~ thereo-~ are rather complicated, the powders thus produced are not stable, being reduced much in magnetic characteristics by pressure and heat and being poor in print-through characteristics. These defects may be explained by the phenomena that the crystal magnetic aniso-tropy of 3 axes directions of the cubic lattice becomes dominant due to the diffusion of Co2+ ion entered into 16d site of the spinel crystal structure.
The assignee of this application filed a patent appli-cation on a novel method free from the prior art defects which was laid open as Japanese patent application publication No.
10994/73, which is now considered effective. The method of this publication is generally referred to as a cobalt hydroxide adsorbing method, in which cobalt hydroxide is adsorbed on the surface of acicular goethite, y-Fe2o3 or Fe3O4 and when the nucleus is y-Fe2O3 an acicular magnetic material of high coer-cive force can be obtained only by subjecting the same to asuitable thermal heating process. According to various experi-ments, however, when r-Fe2O3 adsorbed with cobalt hydroxide is subjected to a thermal treatment or processing at, for example, high temperature (400C) to diffuse in the particles thereof Co2+ ion, the coercive force increases remarkably but the insta-bility of its magnetic characteristics becomes great and its print-through property becomes worse than that of the original material. Further, even when Co2+ ion is not diffused in the particles, when CO-ferrite appears on the surface of the particle, the print-through property is also deteriorated.
It is also known that iron oxide powders adsorbed with 3Gi~9 - cobalt hydroxide as such are used in the magnetic medium. Iron oxide magnetic powders of this type are disclosed in the Japanese Publication Nos. 74399/74 (Toda Kogyo Ltd., Co.), 74400/74 (Toda Kogyo Ltd., Co.), 113159/74 (Tokyo Denki Kagaku Kogyo Ltd., Co.) and so on. According to the methods of making iron oxide magnet-ic powers disclosed therein, alkali is added to an aqueous solu-tion of cobalt salt, into which iron oxide powders are dispersed, to deposit a cobalt compound on the surface of iron oxide pow-ders. In this case, the concentration of alkali is lower than 3 mol/~e. The thus obtained iron magnetic powders have the depos-ited cobalt hydroxide on their surface, so that they have a large hydrophilic property due to OH radicals on their surface. Thus, their dispersion property in organic solvents is low which is necessary during magnetic paint manufacturing process in magnetic recording medium making process, and hence the rectangular ratio Br/Bm of the magnetic recording medium is low. Further, even if the iron oxide magnetic powder which is made by the above method is subjected to thermal treatment to decompose the cobalt hydroxide on the surface of powder and to decrease the hydro-philic property on the surface and hence to increase the disper-sion property in the paint, the increase in dispersion property is not sufficient when the alkali concentration is lower than 3 mol/~, because the deposition of cobalt compound on the surface of iron oxide powder is not homogeneous. Further, the iron oxide magnetic powder, on which a cobalt compound is deposited from a solution whose alkali concentration is lower than 3 mol/~, is lacking in improvement of its coercive force Hc with respect to the depositing amount of cobalt compound. If high coercive force is desired to be obtained, it is necessary to deposit a large amount of cobalt compound, which results in the iron oxide magnetic powder being lowered in magnetization degree and hence ' ' . . ., - ~
3~
a magnetic recording medium made by using such a powder is low in output.
OBJECTS AND SUMMA:RY OF THE INVENTION
The present invention has as an object to provide a method of making a magnetic iron oxide powder which is high in coercive force for use with a novel magnetic recording medium which is improved in print-through characteristic and high in rectangular ratio Br/Bm.
It is another object of the invention to provide a method of making a magnetic iron oxide powder in which iron oxide is dispersedin an aqueous solution of cobalt salt, alkali is added to this solution to such an extent that the alkali con-centration of the liquid phase the reacted dispersion is higher than 3 mol/Ae but lower than 12 mol/A~ and the dispersion is heated to deposit~a cobalt compound on the surface of iron oxide particles to modify the properties thereof~
A further object of the invention is to provide a method of making magnetic iron oxide particles in which the magnetic iron oxide powder having deposited thereon the above modified cobalt compound is subjected to thermal treatment in a non-reduction atmosphere at a temperature between 100C and 200C.
According to an aspect of the present invention there is provided a method of making magnetic iron oxide powder which comprises the steps of dispersing iron oxide particles into an aqueous solution of a cobalt salt, adding alkali to said aqueous solution to such an extent that the alkali concentration of the liquid phase of the reacted dispersion becomes higher than 3 mol/~ but lower than 12 mol/~ , and heating the resultant dis-persion so as to modify said iron oxide particles with cobalt.
The other objects, features and advantages of the ~ 3~6i9 present invention will become apparent from the following de-scription taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF T~E DRAWINGS
Figs. 1 to 5 are graphs showing the relations between the coercive force of magnetic powders and their thermal treat-ment or processing time used for explaining the method of the present invention;
Fig. 6 is a graph plotting the coercive force against the atomic ratio of cobalt to iron of magnetic powders which are used for the explanation of the invention;
Fig. 7 is a graph showing the relation between the coercive force of magnetic powders and their thermal processing temperature;
Fig. 8 is a graph showing the relation between the rectangular ratio of a magnetic tape and the thermal processing temperature of magnetic powders; and Fig. 9 is a graph showing the relation between the print-through value of a magnetic tape and the thermal processing ` temperature.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method of the present invention will be described hereinbelow. According to the present invention, acicular iron oxide particles such as r-Fe2O3, Fe3O4 and a substance whose oxidized state is therebetween, i.e., O~Fe2+ /Fe3+ ~0.5 are dispersed into an aqueous solution of cobalt salt; alkali is added to this aqueous solution to such an amount that the alkali concentration of the liquid phase of the resultant reaction dis-persion becomes more than 3 mol/~but less than 12 mol/A?; and the resultant reaction dispersion is heated to modify the acicular iron oxide particle with cobalt. That is, the surface of the acicular iron oxide particle is coated with a cobalt compound ~3q~
consisting of cobalt hydroxide, cobalt oxide or an intermediate product. Then, the reaction compound is washed by water, dried at a predetermined temperature, rinsed with water again, and dried again. Thereafter, the product is subjected to a heating process at a predetermined temperature in non-reducing atmos-phere to produce desired magnetic powders.
In case of making the magnetic powders, it is desired that the mixing ratio of acicular iron oxide particles with cobalt salt be such that the atomic ratio of cobalt with iron, Co/Fe, of the resultant particle is within the range between 0.1 and 10 at. %. As the aqueous solution of cobalt salt, an aqueous solution of cobalt chloride, cobalt bromide, cobalt sulfate, cobalt nitrate, cobalt acetate, or mixtures of two or more of the foregoing or the like can be employed. Also, the alkali which is used in the present invention is a strong alkali or hydroxide of alkali metal such as lithium hydroxide, sodium hydroxide, potassium hydroxide, or mixtures of two or more of the foregoing or the like. The alkali concentration of the liquid phase of the resultant reaction dispersion, which consists of the aqueous solution of cobalt salt dispersed with acicular iron oxide particles and treated with alkali, is select-ed between 3 mol/~ and 12 mol/~, as set forth previously, and more desirably between 3.5 mol/~ and 10 mol/~. When the said alkali concentration becomes lower than 3 mol/~, the coercive force Hc becomes low, the dispersion property is deteriorated, and the rectangular ratio of a magnetic recording medium using the par-ticles is lowered. While, when the said alkali concentration exceeds 12 mol/~, the surface of the iron oxide particle is partially dissolved and its shape is deformed, which results in that the increase of coercive force Hc is ended and the rectangu-lar ratio becomes low. In this case, the alkali concentration is defined as the concentration of the ion pair of alkali metal ' -6-~ ., ~3Ç~
atom cation and hydroxy ion in the liquid phase of the resultant reaction dispersion.
When the acicular iron oxide particles are dispersed into the aqueous solution of cobalt salt, it is desired that the acicular iron oxide particles be dispersed into an aqueous solution of cobalt salt whose weight is same as or greater than that of the acicular iron oxide particles and that the aqueous solution of cobalt salt have a concentration lower than 1.5 mol/~, more desirably lower than 1.0 mol/~ so as to deposit a cobalt compound homogeneously on the surface of the acicular iron oxide particles. The heating temperature of the dispersion is generally desired to be between 70C and the boiling point thereof, and as the atmosphere outside the dispersion, a non-oxidizing atmosphere such as nitrogen, argon, mixture of the foregoing or the like can be used, but an oxidizing atmosphere such as oxygen, air, or a mixture of the foregoing with nitrogen or argon or the like therewith are more desirable.
The acicular iron oxide particles having a cobalt compoun~
deposited thereon are washed with water, dried at a predetermined temperature, or after being washed with water and dried, they are subjected to a heating process at a predetermined temperature in non-reducing atmosphere to achieve desired magnetic powders or parti-cles. In this case, the range of temperatures at which the particles, which have been dried and are subjected to the heating process in non-reduction atmosphere, is desirably between 100C
and 200C, more desirably between 120C and 200C. When the temperature exceeds 200C, the coercive force Hc of magnetic particle is lowered, while when the temperature becomes lower than 120C and further lower than 100C, the dispersion property thereof becomes poor. The print-through characteristic is good at the temperature range between 100C and 200C, more desirably ~3~
between 120C and 200C, and becomes poor when outside the above temperature ranges. The heating or thermal process time in non-reducing atmosphere is required more than 0.5 hours, but if the heating process time is carried out in more than 5 hours at the high temperature side (~200C), the coercive force Hc becomes low, which is not desired. As the atmosphere for the thermal process, oxidizing, inert and reducing atmospheres may be used, but if the coercive force Hc, dispersion property, print-through characteristic and so on are taken into account, oxidizing and inert atmospheres are desired. Especially, in order to change Co(OH)2 on the surface of the magnetic powder (master powder) to oxide easily and stably without causing any lowering of the magnetic characteristic, an oxidizing atmosphere is most desired.
If a reducing atmosphere such as hydrogen gas is employed, cobalt ferrite appears partially and hence the print-through characteristic is deteriorated. In the case of inert atmosphere i.e. nitrogen gas, when compared with air, the dispersion proper-ty i.e. rectangular ratio Br/Bm and print-through value are lowered somewhat but are better than those of the prior art.
In the case that alkali is mixed into the aqueous solution of cobalt salt, into which iron oxide magnetic particles are dispersed, when the mixture is heated to deposit cobalt com-pound on the surface of the magnetic powders and magnetic powders of high coercive force are made, it is generally said that as the amount of added cobalt is great, the coercive force of mag-netic particles is high. However, the inventors of the present invention have found that the rectangular ratio Br/Bm of a magnetic tape using the magnetic powder, which is prepared under the condition of an alkali concentration between 3 mol/~ and 12 mol/~e regardless of the amount of added cobalt, is improved.
As set forth above, according to the invention, the magnetic record~ng med~um which is high in coercive force, superior in dispersion property and good in print-through effect is made. In the invention, even if Co2+ ion is not diffused into iron oxide particles, the coercive force Hc becomes high. The reason for this may be that some magnetic interaction appears on the boundary between the surface of the iron oxide particle and the adsorbed substance i.e. cobalt oxide (surface magnetic anisotropy).
Hereinbelow, the present invention will be described further with reference to Examples.
Example 1 Acicular y-Fe2O3 particles (whose coercive force Hc is 380 e~ whose longer axis is 0.5 ~m (micrometer) and whose axis ratio is about 8) in an amount of 3 Kg had been dispersed into 20Q of water, 2Q of cobalt salt aqueous solution in which 894g of CoCQ2 6H2O had been dissolved, was added to the former, and the resultant mixture was stirred sufficiently, which means that Co was added at 10 at. ~ (Co/Fe ratio).
Then, 8Q of alkali aqueous solution in which 3.8 Kg of NaOH
had been dissolved was added to the above dispersion. The resultant dispersion was in an aqueous medium which had a strong alkali concentration of about 3 mol/Q. The final dispersion was heated at 100C for 4 hours while being stirred sufficiently. After the heating, the dispersion was rinsed with water to have a neutral pH, and filtered with a suction filter. Thus, ~-Fe2O3 particles, each having Co on the surface thereof, were produced. These particles were dried and then subjected to a thermal processing at 100C in air for 5 hours. The magnetic characteristics of magnetic particles thus produced were as follows;
Saturated magnetization as = 71.8 emu/g Coercive force Hc = 675 e g _ 3~
Ratio ~r (residual magnetism~/sS = 0.48 The thus produced iron oxide powders, each con-taining Co, were mixed with the following composition for about 48 hours in a ba~l mill to produce magnetic paint.
Magnetic iron oxide powders 100 wt. parts containing Co Vinyl chloride-vinyl acetate 17.5 wt. parts copolymer (Binding agent)(VAGH: Trade name of UCC Ltd, Co.) Polyurethane Resin 7.5 wt. parts (Binding agent)(Estane 5702: BF Goodrich Chemical Co.) Lecithin (Dispersion agent) 2.0 wt. parts Methyl-ethyl ketone (Solvent) 100 wt. parts Cyclohexanone ~Solvent) 100 wt. parts The above magnetic paint was coated on a film, which is made of polyethylene terephthalate with 12 ~m in thickness such that the thickness of the paint after being dried is 6 ~m. Thus, a magnetic tape was produced. In this case, the coercive force Hc thereof was 660 e and the rectangular ratio Br/Bm thereof was 0.79.
Figs. 1 to 5 are graphs, respeativel~, showing the relations between the coercive force Hc of the magnetic powder and vary;ng Co adsorbing conditions.
Fig. 1 is a graph showing the relation between the coercive force Hc of the magnetic powder and the heating time (hr) of the reaction dispersion under the condition that the added amount of Co was 1 at. % and NaOH concentrations of the reaction dispersion (1 mol/Q, 3 mol/Q, 5 mol/Q, 10 mol/Q
and 15 mol/Q) were varied.
Figs. 2 to 5 are graphs showing the characteristics 3Q similar to that of Fig. 1 when the added amount of Co was 3 at. %, 5 at. %, 10 at. %, and 15 at. %, respectively. In the respective graphs, the marks X,~;O ,~ and ~ show the cases -- 10 _ ~ ~3~
of NaOH concentration of 1 mol~, 3 mol/Q, 5 mol/Q, 10 mol/Q
and 15 mol~Q, respectively.
The following Tables I, II, III and IV are respectively made from the characteristic graphs of Figs. 1 to 5 and show the changes of the coercive forces and rectangular ratios of magnetic tapes which are prepared by using the magnetic powders- made by varying one of Co adsorbing con-ditions.
3~
Table I
_ Co absorbing condition Concent- Adding Heating Heating Coercive Rectan-ration of amount temper- time force of gular NaOH of Co ature tape ratio (at.~, of tape (mol/") Co/Fe) (C) (hour) (e) (Br/Bm) Comparison 1 10 100 4 528 0.73 _ __ _ Example 3 10 100 4 660 0.79 Example 5 10 100 4 710 0.80 . _ 3 10 10 100 4 750 0.81 .
~ Dn ; 15 ~ ~ ~ o l o ~l ~3~
,:
~able II
. . Co absorbing condition Concent- Adding Heating Heating Coei-cive Rectan-ration amount temper- time force of gular of NaOH of Co ature tape ratio (at. %, of tape (mol/Q) Co/Fe) (~C) (hour) (e) (Br/Bm) ._.. _ . Example 3 1 100 1 470 0.79 _ _ _ .~ _. _. _ .
Example 3 5 100 1 575 0.79 ... __ .....
6 3 10 100 1 608 0.79 . __ ..
Comparison 1 1 100 1 447 0.72 .
Comparison . 4, 1 5 100 1 488 0.74 . .
_ _ _ __ Comparison 1 10 100 1 491 0.73 .
~ 13 -1~3069 Table III
.~
Co absorbing condition Concent- Ac-ding Heating Heating Coercive Rectan-ration amount temper- time force of gular of NaOH of Co ature tape ratio (at.%, of tape (mol/Q) Co/Fe) (C) (hour) (e) (Br/Bm) _ _ Example 35 100 1 575 0.79 _ _ __ _ _ .
Exa8mPle 3 5 100 4 658 0.80 .. __ _ _ _ Example 3 5 100 24 690 0.82 .. .; _ . . . . _ ._ ._ Comparison 1 5 100 1 488 0.74 _ _ I .
Comparison 7' 1 5 100 4 513 0.75 .. .. _ . . ._ . .
Comparison 1 5 100 ~ 549 0.76 ~3~69 Table IV
Co absorbing condition . Concent- Adding Heating Heating Coercive Rectangu-ration amount temper- time force of lar rati of NaOH o(atC,~o, ature tape of tape (mol/Q) Co/Fe) (C) (hour) (e) (Br/Bm) Comparison 1 10 100 24 580 0.74 _ Example 3 3 100 4 576 0.80 ~
_ ...
Example 5 3 100 1 580 0.81 Example 10 3 100 0.5 586 0.81 _ Comparison15 3 100 0.5 570 0 76 3~
Table I shows the characteristics of a tape which is made by varying the amount of sodium hydroxide in aqueous medium, with all other conditions being held constant.
In Table I, Examples 1, 2 and 3 are the cases that the NaOH concentration is at 3 mol/Q, 5 mol/Q and 10 mol/Q, respectively, and Comparisons 1' and 2' are the cases that the NaOH concentration is at 1 mol/Q and 15 m~l/Q, respectively.
Further, in Table I the reason why the rectangular ratio of Comparison 1' in which NaOH concentration 1 mol/Q, is low may be explained that under this condition Co is not adsorbed on the surfaces of magnetic powders homogeneously, and the reason why the rectangular ratio of Comparison 2', in which NaOH concen-tration is selected 15 mol~Q, is low may be considered that under this condition the magnetic powders are partially dissolved due to the high alkali content and hence their physical forms are deformed or poor. In this case, it is ascertained that even ~f the concentration of NaOH in the reaction solution was higher than 12 mol/Q, no magnetic powders of higher coercive force could be obtained.
The above Table II shows the characteristics of respective tapes in which the amounts of Co and NaOH were varied but the other conditions were kept constant. In Table II, the Examples 4, 5 and 6 show the cases that the NaOH concentration is held constant at 3 mol/Q but the adding amount of Co is varied 1 at. %, 5 at. % and 10 at. %, respectively, while Comparisons 3', 4' and 5' show the cases that the NaOH concentration is held constant at 1 mol/Q but the adding amount of Co is selected 1 at. ~, 5 at. % and 10 at. % respectively. From Table II it is noted that, regardless of the added amount of Co, in case of making magnetic powders with a NaOH concentration higher than 3 mol/Q, the rectangular ratio of a magnetic tape increases.
3~
The above Ta~1e III shows the characteristics of a magnetic tape in which the heating time of the reaction dispersion and NaOH concentration therein are varied but the other conditions are kept the same. In Table III, Examples 7 (~which is the same as Example 5), 8 and 9 show the cases that NaOH concentration is held constant at 3 mol/Q and the heating time is 1, 4 and 24 hours, respectively, while. Comparisons 6' (which is the' same as Comparison 4'), 7~ and 8' show the cases that NaOH concentration is held constant at 1 mol~Q and the heating time is 1, 4 and 24 hours, ' '.
respectively. From Table III it will be noted that, in the case where magnetic powders are made wi.th an NaOH concentration higher tI.an 3 mol~Q, the rectangular ratio of the magnetic tape increases regardless of the heating time.
When such a relation between NaOH concentration and the coercive force of produced magnetic powders is con-sidered, as NaOH concentration becomes lower than 3 mol/Q, the coercive force of the produced magnetic powders becomes low, and also even if the added amount of Co is increased and the heating time is increased under the same condition, the coercive force is not increased. Accordingly, in order to obtain a coercive force higher than a coercive force HC-600 e' it is necessary to use a NaOH concentration higher than 3 mol/Q.
Further, even if the magnetic powder of the required coercive force can be produced with a NaOH concentration of lower than 3 mol~Q, it is hetter, in view of the rectangular ratio of magnetic tape, to reproduce under the condition of NaOH concentration higher than 3 mol/Q, selecting the condition of the adding amount of cobalt and the heating time.
The above Table IV shows the rectangular ratios of magnetic tapes using magnetic powders with the coercive ~ 3~
force of about 580 e which are made by varying NaOH con-centrations. In Table IV Comparison 9', Examples 10, 11~
12, and Comparison 10' are the cases that NaOH concentration was 1 mol/Q, 3 mol~Q, 5 mol~Q, 10 mol~Q and 15 mol/Q, respectively. From Table IV, it will be noted that the magnetic powders, wh~ch are produced with a NaOH concentration 3 to 10 mol/Q, have good rectangular ratios when they are used to form magnetic tapes and such magnetic powders can be produced with smaller amounts of cobalt and shorter heating times. As described above, in order to improve the rectangular ratio of a magnetic tape, it is preferred that NaOH concen-tration during coba;lt adsorption is within a range of 3 mol/Q
to 12 mol~Q.
Fig. 6 is a graph showing the coercive force of magnetic powders at respective Co adding amounts with varying NaOH concentrations. The graph of Fig. 6 is prepared from those of Figs. 1 to 5 in which only their maximum values are extracted and in which the values inscribed in the vicinit~ of the respective marks represent the heating time in hours. From the graph of Fig. 6 it will be noted that if NaOH concentration is maintained within the range of the present invention the coercive force increases and if the added amount of Co is lower than 10 at. % in the atomic ratio of Co~Fe, the improved results are achieved.
Example 13 3 Kg of acicular ~-Fe2O3 powders or particles (having a coercive force HC=380 e' long axis of 0.5 ~m and axis ratio of about 8) had been dispersed into 20Q ot water, aqueous solution of 2Q into which 447 g of CoCQ2-6H2O
(on market) had been dissolved was added to the former and then the mixture dispersion was stirred sufficiently, which resulted in a dispersion which contained Co at 5 at. % (in 3q~
Co/Fe atomic .ratio~. Then, 8Q of aqueous solution into which6.0 Kg of NaOH has l~een dissolved was added to the above resultant dispersion, so that the finally resultant suspension had a solution of strong alkali of about 5 mol/~. This final dispersion was heated at 100C for about 1 hour while being stirred sufficiently. After heating, the solution was rinsed with water to be neutral in pH, y-Fe2O3 particles on which cobalt hydroxide was deposited were extracted by means of a suction funnel, and then the y-Fe2O3 particles were dried. The 10 magnetic characteristics of thus obtained magnetic particles were such that saturation magnetization ~s was 73.3 emu/g, coercive force Hc was 606 e and ~r (residual magnetization)/aS
was 0.48, respectively. These magnetic particles will be referred as a master powder A.
The master powder ~ was subjected to thermal processing in air with the temperature being changed from 70C
to 400C. Then, when the surface condition of the magnetic particle or Co adsorption condition was analyzed by the electron ray d~ffraction and X-ray photoelectron spectrometry 20 (ESCA), the following Table V was obtained.
Table V
State of Co adsorbed on magnetic particle Thermal processing (8y Electron ray difraction and condit'ion ' ' X-'ray photoelectron spectrometry~
70C - 20 hours Co(OH)2 100C - 1 hour Co(OH~2 130C - 1 hour CoOOH, Co3O4 150C - 1 hour CoOOH, Co3O4 150 C - 5 hours 3O4 200C - 1 hour Co3O4 370C - 1 hour C3O4, CoFe2O4 400C - 1 hour CoFe2O4 ~3~
According to this Table V, when the temperature is lower than 100C, Co~OH~2 on the surface of the master powder A is still as it is or not changed. Butf when the temperature goes up to 130C, it is observed that CotOH)2 is changed into Co3O4. It is ascertained that as the temperature becomes higher, Co is gradually diffused into the master powder A.
The following Tables VI and VII respectively show the results when the master powder A was subjected to the thermal processing in nitrogen and hydrogen atmospheres and a similar analysis was carried out on the resultant product. According to the thermal processing in nitrogen, it is noted that the adsorbed cobalt hydroxide is changed over about 130C, and as the temperature of the thermal processing is raised further, Co starts its diffusion into the magnetic powder. While, according to the thermal processing in the hydrogen atmosphere, it is noted that Co starts its diffusion into the magnetic powders at the temperature of about 200C and at the same time the master powder starts to be reduced.
Fig. 7 is a graph showing the measured results of a coercive force Hc f the magnetic powder after the similar thermal processing has been carried out for about 1 hour. In the graph of Fig. 7, a curve I connecting the marks O , a curve II connecting the marks X and a curve III connecting the marks ~ show the thermal processes in the atmospheres of air, nitrogen and hydrogen, respectively.
It is noted in the thermal processes of air and nitrogen atmospheres that the coercive force Hc is lowered considerably within the temperature range of 200C to 350C, which may be due to the fact that as the temperature of the thermal process becomes high, Co2~ ion is caused to be moved and the ~3~
structure of the interface between the magnetic particle and the substance adsor~ed thereon, which interfacial structure causes the increase of t~e coercive force Hc, will disappear.
As the temperature of the thermal process becomes higher, the coerci~e force Hc again increases, which is caused by the fact that Cois diffused into the magnetic powders. Since the similar diffusion of Co occurs at relatively low temperature in the thermal process in the hydrogen atmosphere, it may be considered that no temperature range within which the coercive force Hc is lowered has been observed.
~ .
'~
3~
Table VI
State of Co adsorbed on magnetic Thermal processing (By electron-ray diffraction condition and X-ray photoelectron spec-trometry) ... ..
130C - 1 hour CoOOH, Co(OH)2 150C - 1 hour CoO, CoOOH
150C - 5 hours CoO
200C - 1 hour CoO
300C - 1 hour CoO, CoFe2O4 400C - 1 hour _ .
Table VII
. .
State of Co adsorbed on magnetic Thermal processing (By electron-ray diffraction conditlon and X-ray photoelectron spec-trometry) . . _ __.
150C - 1 hour Co(OH)2 200C - 1 hour CoO, CoFe2O4 300C - 1 hour 2 4 ,~ .
~ ~, ~i3~
Fig. 8 is a graph showing the results obtained when the magnetic powders, which are subjected to the thermal processes in the above respective atmospheres, are used to provide magnetic tapes and the rectangular ratios (Br/Bm) of the respective magnetic tapes are measured. In the graph of Fig. 8, a curve rv connecting the marks ~, a curve V connect-ing the marks ~ and a curve VI connecting the marks ~ represent the cases of the thermal processing in air, nitrogen and hydro-gen atmospheres, respectively. In this case the magnetic tape is made by the same manner as that of Example 1.
From the graph of Fig. 8 it is noted that the cobalt oxides derived from cobalt hydroxide which is coated on the surface of the master powder is superior in the dis-persion property.
Fig. 9 is a graph showing themeasured print-through values of the magnetic tape having the characteristics of Fig. 8 according to JISC-5542. In the graph of Fig. 9, a curve VII connecting the marks O , a curve VIII connect-ing the marks ~ and a curve IX connecting the marks ~ re-present the thermal processes in air, nitrogen and hydrogen atmospheres, respectively. It is noted that in air and nitrogen atmospheres, the print-through value begins to be improved with the thermal processing at temperature higher than 100C and further about 120C. While it is noted that when the temperature of the thermal processing becomes high, the print-through value is deteriorated due to the diffusion of Co into the master powder. In the graph of Fig. 9, the print-through value of the tape lower than -50 dB can not be used as a practical tape.
From the above resul~s, it will be understood that magnetic particles, which will represent superior characteristics when coated on a tape base, are obtained in the case that they are subjected to the thermal processing in air or inert gas at the temperaturé range between 100C and 200C, and preferably 120C and 200C.
It is also possible that even if magnetic particles of Fe3O4 or a substance whose oxidizing condition is be-tween Fe3O4 and Fe2O3 (intermediate substance), are used, such magnetic particles whose surface is covered by cobalt oxide can be made and hence a magnetic tape having improved characteristics is obtained.
Example 14 2 Kg of magnetic particles, whose divalent to triva-lent iron ratio Fe2~Fe3~ is 0.20 (which have the coercive force Hc of 260 e~ long axis of 0.5 ,um and axis ratio of about 8) had been dispersed into 20Q of water, 2Q of aqueous solution into which 300g of CoCQ2 6H2O had been dissolved was added to the dispersion, and the resultant suspension was stirred sufficiently. Then, 8Q of aqueous solution into which 6.0 Kg of NaOH had been dissolved was added to the dispersion, and the resulting dispersion was heated at 100C
for 1 hour while being stirred sufficiently. The magnetic characteristics of the particles are as follows.
~s = 80.2 emu/g Hc = 576 e ar~aS = 0.46 This magnetic powder will be referred as a master powder B.
The following Table VIII shows the characteristics of the master powder B after being subjected to thermal processing at 150C for 1 hour in air and those of a magnetic tape which is made by using thus prepared magnetic powders in the manner as recited in Example 1. From Table VIII
it is noted that the characteristics of the tape are improved by the thermal processing.
Example 15 3 Kg of ~-Fe2O3 particles (having a coercive force Hc = 405 e~ long axis = 0.4 ,um and axis ratio of about 8) had been dispersed into 20Q of water, aqueous solution into which 268g of CoCQ2-6H2O (on market) had been dissolved was added to the dispersion, and the resulting dispersion was stirred sufficiently, which results in that about 3 at. ~ (Co/Fe atomic ratio) of Co was added. Then 8Q of aqueous solution into which 4.2 Kg of NaOH had been dissolved was added to the above dispersion. Thus, the finally resultant liquid phase had a strong alkali content of about 3.5 mol/Q.
This resulting dispersion was heated at 100C for 1 hour while being stirred sufficiently. Thus prepared magnetic powders have the magnetic characteristics that their ~s = 74.6 emu/g, Hc = 587 e and ~r~s = 0.48. These magnetic powders will be referred to as a master powder C.
The magnetic characteristics of a magnetic tape which uses magnetic powders prepared by subjecting the master powder C to the thermal processing in the manner similar , .
l~C3~
to that of Example 1 are shown in the following Table IX.
Table IX
Thermal process~ng Coercive Rectangular Print-through condition force of ratio of value of magnetic tape tape powd~Or~ Br/B (dB) None 569 0.74 -51.1 Air 130C - 1 hour 563 0.83 -56.3 Nitrogen atmosphere 200C - 1 hour 558 0.82 -54.7 From the above Table IX is is ascertained that the characteristics of the magnetic tape are improved by the thermal processing.
Also, according to the method of the present invention the master powders can be made by using substances different from the cobalt salt and alkali used in Example 13.
Example 16 3 Kg of acicular y-Fe2o3 particles (having the coer-cive force = 380 e' long axis = 0.5 ,um and axis ratio = 8) were dispersed into 20~ of water, 2Q of aqueous solution into which 528 g of CoSO4 7H2O (on market) had been dissolved was added to the dispersion, and the resultant dispersion was stirred sufficiently, which meant that Co was added at about 5 at. % (Co/Fe atomic ratio). Next, 8~ of aqueous solution into which 6.3 Kg of LiOH H2O had been dissolved was added to the above aqueous dispersion. Thus, the final dispersion had a strong alkali solution of about 5 mol/Q.
The magnetic characteristics of magnetic powders, which were obtained by processing the above solution in the manner similar to that of Example 13, are as follows. These are ~s = 74.2 emu/g, Hc = 628 e and ~r/~s = 0.48. This magnetic powder will be referred to as master powder D.
, 3~
The characteristics of a magnetic tape, which uses magnetic powders provided by subjecting the master powder D to the thermal processing similar to Example 13, are shown in the following Table X.
Table X
Thermal Processing Coercive Rectangular Print-through condition force of ratio of value of magnetic tape tape H Wd(or) Br/Bm (dB) None 628 0.75 -50.8 Air 160C - 1 hour 619 0.83 -55.1 Nitrogen atmosphere 200C - 0.5 hour 651 0.81 -49.0 From the above Table X it is noted that the dispersion property and print-through characteristics are especially improved by the thermal processing in air.
As described above, according to the method of making magnetic powders of the invention, a magnetic recording medium, which has high coercive force and superior disper-sion property and Is superior in print-through characteristic, can be made. This magnetic medium is preferred for use as a high density recording tape and so on.
It will be apparent that many modifications and variations could be effected by one skilled inthe art without departing from the spirits or scope of the novel concepts of the present invention so that the scope of the invention should be determined by the appended claims only.
Claims (19)
1. A method of making a magnetic powder comprising the steps of:
mixing iron oxide particles with an aqueous solution of a cobalt salt, adding alkali to the mixture to produce a dis-persion having an alkali concentration of at least 3 mols per liter but not more than 12 mols per liter, and heating the suspension to cause deposition of a cobalt compound on the surfaces of the iron oxide particles.
mixing iron oxide particles with an aqueous solution of a cobalt salt, adding alkali to the mixture to produce a dis-persion having an alkali concentration of at least 3 mols per liter but not more than 12 mols per liter, and heating the suspension to cause deposition of a cobalt compound on the surfaces of the iron oxide particles.
2. A method according to claim l in which said iron oxide is gamma-ferric oxide, Fe3O4, or an intermediate iron oxide having a ratio of ferrous to ferric ions greater than 0 but less than 0.5.
3. A method according to claim 1, wherein said alkali is selected from one of lithium hydroxide, potassium hydroxide, sodium hydroxide and mixture thereof.
4. A method according to claim 1, wherein said heating step is carried out at temperature higher than 70°C.
5. A method according to claim 1, wherein said cobalt salt is selected from one of cobalt chloride, cobalt bromide, cobalt sulfate, cobalt nitrate, cobalt acetate and their mixture.
6. A method according to claim 1, wherein said iron oxide particles are dispersed into aqueous solution of cobalt salt more than the former in weight.
7. A method according to claim 1, wherein said aqueous solution of cobalt salt is selected lower than 1.5 mol/? in concentration.
8. A method according to claim 1, further comprising steps of drying and thermal-processing resultant powders.
9. A method according to claim 8, wherein said thermal processing is carried out in non-reduction atmos-phere and at a temperature range of 100°C and 200°C.
10. A method according to claim 8, wherein said thermal processing is carried out in non-reduction atmos-phere and at a temperature range of 120°C and 200°C.
11. A method according to claim 10, wherein said thermal processing is carried out more than 5 hours.
12. A method of making a magnetic powder comprising the steps of:
mixing acicular magnetic iron oxide particles with an aqueous solution of a cobalt salt which is lower than 1.5 mols per liter in concentration, said solution being substantially free of iron ions, adding an alkali metal hydroxide to the mixture to produce a dispersion having an alkali concentration of at least 3.5 mols per liter but not more than 10 mols per liter, heating the dispersion at a temperature higher than 70°C to cause deposition of a cobalt compound on the surfaces of the acicular magnetic iron oxide particles, the atomic ratio of cobalt to iron being within the range of 0.1 and 10 atomic percent, and drying and heat treating the powder in a non-reducing atmosphere at a temperature in the range of 100° to 200°C.
mixing acicular magnetic iron oxide particles with an aqueous solution of a cobalt salt which is lower than 1.5 mols per liter in concentration, said solution being substantially free of iron ions, adding an alkali metal hydroxide to the mixture to produce a dispersion having an alkali concentration of at least 3.5 mols per liter but not more than 10 mols per liter, heating the dispersion at a temperature higher than 70°C to cause deposition of a cobalt compound on the surfaces of the acicular magnetic iron oxide particles, the atomic ratio of cobalt to iron being within the range of 0.1 and 10 atomic percent, and drying and heat treating the powder in a non-reducing atmosphere at a temperature in the range of 100° to 200°C.
13. A method according to claim 12 in which said acicular magnetic iron oxide is gamma-ferric oxide, Fe3O4, or an intermediate iron oxide having a ratio of ferrous to ferric ions greater than 0 but less than 0.5.
14. A method according to claim 12, wherein said alkali metal hydroxide is selected from the group consisting of lithium hydroxide, potassium hydroxide, sodium hydroxide, and mixtures thereof.
15. A method according to claim 12, wherein said cobalt salt is selected from the group consisting of cobalt chloride, cobalt bromide, cobalt sulfate, cobalt nitrate, cobalt acetate, and mixtures thereof.
16. A method according to claim 12, wherein said acicular magnetic iron oxide particles are dispersed into said aqueous solution of cobalt salt weighing more than the acicular magnetic iron oxide particles.
17. A method according to claim 12, wherein said aqueous solution of cobalt salt is less than 1.0 mol per liter in concentration.
18. A method according to claim 12, wherein said non-reducing atmosphere is selected from the group consisting of nitrogen, argon, oxygen, air, and mixtures thereof.
19. A method according to claim 12, wherein said thermal processing is carried out in a non-reducing atmosphere at a temperature in the range from 120 to 200°C.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP79645/77 | 1977-07-04 | ||
| JP7964577A JPS5413997A (en) | 1977-07-04 | 1977-07-04 | Magnetic recording medium and its manufacture |
| JP67490/78 | 1978-06-05 | ||
| JP53067490A JPS6034805B2 (en) | 1978-06-05 | 1978-06-05 | Manufacturing method of magnetic iron oxide powder |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1103069A true CA1103069A (en) | 1981-06-16 |
Family
ID=26408711
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA306,504A Expired CA1103069A (en) | 1977-07-04 | 1978-06-29 | Method of making magnetic powders |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1103069A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5650131A (en) * | 1993-11-01 | 1997-07-22 | Minnesota Mining And Manufacturing Company | Process for making goethite |
| CN113436824A (en) * | 2021-07-07 | 2021-09-24 | 上海圣石生物医学科技有限公司 | Magnetic wave-absorbing material, preparation method, application and health-care product thereof |
-
1978
- 1978-06-29 CA CA306,504A patent/CA1103069A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5650131A (en) * | 1993-11-01 | 1997-07-22 | Minnesota Mining And Manufacturing Company | Process for making goethite |
| CN113436824A (en) * | 2021-07-07 | 2021-09-24 | 上海圣石生物医学科技有限公司 | Magnetic wave-absorbing material, preparation method, application and health-care product thereof |
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